The present invention relates to a pattern inspection device and a pattern inspection method for semiconductors and the like, and, more particularly, to a pattern inspection device and a pattern inspection method which are capable of high-speed inspection of semiconductors and the like.
In a semiconductor manufacturing process, a number of pattern formation steps are repeated. In the respective steps, if manufacturing conditions are not optimized, abnormalities such as foreign substances and defects and the like occur in a circuit pattern of a semiconductor device formed on a substrate. Accordingly, in the manufacturing process, it is necessary to detect the occurrence of an abnormality at an early stage and feed it back to the process.
Generally, in the manufacturing process for an VLSI or the like, as a number of chips having the same circuit pattern are obtained from one sheet of semiconductor substrate, a pattern abnormality is detected by comparing identical circuit patterns of different chips. In an inspection device to inspect a circuit pattern of a semiconductor wafer by using an electron beam, as it takes an enormous amount of time to inspect the entire wafer with the electron beam, a method to compare identical circuit patterns of different chips, by a construction having two electron-optic systems is especially disclosed in Japanese Published Unexamined Patent Application No. Sho 59-6537.
As is well-known in the art, if a difference signal of detection signals, obtained from identical circuit patterns of different chips, exceeds a reference value, it is determined that a pattern abnormality exists. However, in this construction, although it can be determined that an abnormality exists in one of the chips, the chip having the abnormal pattern cannot be determined. In the abnormality determination, comparison with another pattern obtained from another chip is necessary. For this purpose, all the image data of the two chips is stored into an image memory, then the inspection moves to another chip, the electron beam is emitted on the corresponding same pattern, and a determination is made. Accordingly, a large capacity image memory is required, and further, there is a possibility that the stability of the system is impaired with the lapse of time during movement to the other chip.
Further, to improve the throughput of the inspection time, it is necessary to emit a fine electron probe beam of large current on a sample. For this purpose, the brightness of an electron beam source must be high, so that a field-emission type electron source is indispensable as the electron beam source. Note that to operate the field-emission type electron beam source in a stable manner, the degree of vacuum around the electron beam source must be suppressed to the order of 10xe2x88x927 Pa. However, in the conventional construction, it is difficult to closely arrange plural electron-optic systems with maintaining a high vacuum degree in the electron beam guns. For example, in the above-described well-known art, an area around the electron beam source is evacuated from a sample chamber. Further, in a conventional technique described in xe2x80x9cJournal of Vacuum Science and Technology B14(6)xe2x80x9d, page 3776, the entire electron-optical system is placed in one chamber, as shown in FIG. 12. Accordingly, in this construction, to evacuate the area around the electron beam source to a very high degree of vacuum, the sample chamber must be also evacuated to a very high degree of vacuum. However, since a wafer coated with chemical material, such as a resist, emits a large amount of gas, and the structure of a stage to control movement of the wafer is complicated, it is practically impossible to evacuate the sample chamber to a very high degree of vacuum. Generally, the degree of vacuum is merely improved to about 10xe2x88x925 Pa. Even if a very high degree of vacuum in the sample chamber can be realized, as the degree of vacuum in the sample chamber is lowered upon exchange of a sample, the period to evacuate the sample chamber after the sample exchange to the very high degree of vacuum is e.g. equal to or longer than one hour. Thus, it is impossible to inspect a large number of wafers within a short period,
Further, in a construction as shown in FIG. 13 where an electron beam gun chamber 101 and a sample chamber 103 are respectively evacuated by independent vacuum pumps, a large number of vacuum pumps must be provided, with the result that a large number of spaces for placement of vacuum pumps must be provided. For example, in FIG. 13, to provide a vacuum pump for the central electron-optical system, a close arrangement is impossible.
Further, in general detection means, if the electron-optical systems are closely arranged, it is difficult to hold secondary electrons and reflected electrons 302 obtained by emitting an electron beam on a sample within the same electron-optical system. That is, as means for detecting the secondary electrons and reflected electrons 302 in plural electron-optical systems, a method for detection by providing detectors 13 on the rear surfaces of final stage lenses, as shown in FIG. 15, is disclosed in the xe2x80x9cJournal of Vacuum Science and Technology B14(6)xe2x80x9d page 3775. In this construction, it is difficult to hold the secondary electrons and reflected electrons 302 within the same electron-optical system, and so the secondary electrons and reflected electrons 302 are easily attracted by the detector 13 in the adjacent electron-optical system; as a result, precise pattern inspection cannot be made. Further, Japanese Published Unexamined Patent Application No. Hei 2-142045 discloses a method for detecting secondary electrons, accelerated by application of a negative voltage to a sample, that have passed through an objective lens. However, there is no specific construction to improve the efficiency of secondary electron detection.
The present invention has the following construction as means for solving the above problems. That is, at least three electron-optical systems are provided, and detection signals from identical circuit patterns of different chips are compared with each other. If three or more images are obtained at the same time, the position of a pattern defect can be simultaneously determined. Further, in a case where a stage is continuously moved, the same pattern repeatedly exists within the chip, and images continuously obtained by the respective electron-optical systems are sequentially compared, the throughput of the inspection period is improved in proportion to the number of electron-optical systems.
Further, in accordance with the present invention, as shown in FIG. 14, to maintain a high degree of vacuum in an area around the electron source 1, three or more electron-optical systems are provided in one mirror body representing a same column or chamber, and a common vacuum pump evacuates an area around the electron source 1 or an area around an intermediate chamber 102 provided between the electron source 1 and the sample chamber 103, so that the electron-optic systems can be closely arranged. That is, since the areas around the plural electron sources 1 are connected to the area around the sample chamber via fine openings through which electron beams pass, and the areas around the electron sources are evacuated independently of the area around the sample chamber 103, a high degree of vacuum in the areas around the electron sources 1 can be maintained.
Further, to independently detect secondary electrons and reflected electrons, produced in the plural electron-optical systems, in the respective electron-optical systems, the secondary electrons and reflected electrons 302, generated from the sample are accelerated toward the electron source side in the direction of the electron beam axis 9 so that they can be detected by a detector provided toward the electron source side from an objective lens, without colliding against a counter electrode 19, as shown in FIG. 16. The vertical-directional speed of the secondary electrons and reflected electrons-302 on the electron beam axis 9 is constant from a point where they are emitted from the sample. As the speed is accelerated in the direction of electron beam axis 9, the trajectories of the secondary electrons and reflected electrons 302 are directed toward the electron beam axis 9. Then, a voltage U is applied between a sample 10 and the counter electrode 19 opposite to the sample 10, and if the sample 10 is not tilted, but is placed approximately in parallel with the counter electrode 19, an approximately uniform electric field parallel to the sample is distributed between the sample 10 and the counter electrode 19. Then, assuming that it is a parallel electric field, a distance R from a point where an electron is emitted in a direction approximately parallel to the surface of the sample, to a point where the electron reaches the counter electrode is expressed by the following expression, where L is the distance between the sample and the electrode and eV is the energy of electron emitted from the sample.                     R        =                  2          ⁢                                                                      e                  ⁢                                      xe2x80x83                                    ⁢                  V                                                  e                  ⁢                                      xe2x80x83                                    ⁢                  U                                            ⁢                              xe2x80x83                            ⁢              L                                                          (        1        )            
Actually, if the counter electrode 19 has an opening, the electric field is not a parallel electric field around the opening, however, the distance R can be approximated by the expression (1). Considering a scan width S by a primary electron beam on the sample, an area where the electrons emitted from the sample spread at the counter electrode is 2R+S. Accordingly, in a case where the value R is obtained by substituting the maximum energy of the reflected electron i.e. the energy of the primary electron beam into the energy of an emitted electron in the expression (1) as Rmax, and a diameter D1 of the counter electrode is
D1 greater than 2 Rmax+Sxe2x80x83xe2x80x83(2),
the reflected electrons and secondary electrons are not dissipated outward from the counter electrode, and can be collected within the same optical system. Further, in a case where the value R is obtained by substituting 50 eV into the energy eV in the expression (1) as Rse, and a diameter D2 of the opening of the counter electrode 19 is
D2 greater than 2 Rse+Sxe2x80x83xe2x80x83(3),
all the secondary electrons or reflected electrons having an energy equal to or less than 50 eV pass through the opening of the counter electrode toward the electron source side. Note that if the scan width S is sufficiently small, the expressions (2) and (3) can be omitted. If the sizes of the counter electrode and the openings of the counter electrodes are set according to the above-described conditions, the secondary electrons or reflected electrons generated from the sample can be efficiently detected without being dissipated to the adjacent optical system. The secondary electrons and reflected electrons 302 which have passed through the opening of the counter electrode are acted upon by an objective lens 4. The detectors 13 are provided above the objective lens 4, and can detect almost all of the secondary electrons and reflected electrons 302 on trajectories that are changed by the action of the objective lens. As the secondary electrons and reflected electrons 302 are detected in this manner, even in a condition where plural electron-optical systems are closely arranged, the secondary electrons and reflected electrons 302 can be efficiency detected without being dissipated to the adjacent optical system. Further, in a construction where a primary electron beam 301 is almost not deflected, but the secondary electrons and reflected electrons 302 are deflected, e.g., in a construction where a deflector having an intersecting magnetic field and electric field is provided such that the secondary electrons and reflected electrons 302, accelerated in the electron-beam axis direction, pass through the deflector, the secondary electrons and reflected electrons 302 can be more efficiently detected. Further, an opening aperture to limit an opening angle of the primary electron beam is provided toward the electron source side from the detector such that the secondary electrons or reflected electrons do not collide against the opening aperture.
Further, in accordance with the present invention, to avoid any influence by the electromagnetic field produced in each optical system on peripheral electron-optical systems, as shown in FIG. 14, a shielded electrode 17, having a structure to confine the electric field or magnetic field produced in each electron-optical system within the optical system and means to evacuate an electron beam passage to a very high degree of vacuum, is provided in an outer periphery of the electron-optical system, such that an electric field and magnetic field leaking from the electronic lens and the detector can be closed within the same optical system.
In the above construction, as three or more electron-optical systems are closely arranged within the same mirror body or column and a pattern defect is determined in real time, the inspection accuracy can be improved and the inspection speed can be increased in proportion to the number of electron-optical systems. Further, as the degree of vacuum in the areas around the three or more electron sources are maintained at a high degree of vacuum, even in the sample chamber which is in a low vacuum state, e.g., upon sample exchange, the electron sources can be operated in a stable manner. Further, since an electron beam is not deflected from another electron-optical system, a signal detected from a pattern can be independently detected with high accuracy within each electron-optical system. Accordingly, pattern inspection can be performed at a high speed and with high accuracy.